1. Field of the Invention
This invention relates to a process for preparing a foamed molding having, in cross-section, an annual ring pattern by mixing two different resins of the same chemical type, but which possess different physical properties.
2. Description of the Prior Art
Various attempts have been made to prepare synthetic resin moldings having in cross-section an annual ring pattern similar to natural woods, i.e., so-called synthetic woods. Preparation of non-foamed moldings having an annual ring pattern by extrusion or injection molding is relatively easy. However, when it is attempted to prepare foamed moldings having an annual ring pattern by extrusion or injection molding, the annual ring pattern becomes faint because of irregular reflection of pores of the foamed bodies and, therefore, it is difficult to obtain clear annular ring patterns only by combining differently-colored resin materials. This tendency toward faintness of color contrast is especially great when resins of the same chemical type are used. There have been known some processes that attempt to overcome this problem, for example, a process wherein resins of different chemical types (different recurring monomer units) are used and molding is effected in a screw extruder, taking advantage of the incomplete mixing of the resins that occurs due to their incompatibility or the difference in their plasticizing temperatures; a process wherein a rotator is disposed in an extruding nozzle for stirring the material; and a process wherein the resins are melted separately in two extruders and then the annual ring pattern is formed in an extruding nozzle. Alternatively, various attempts have been made in the field of injection molding, for example, employing a plunger-type injector or a screw-type injector.
However, if resins of different chemical types are used, there are disadvantages in that the adhesive bonding strength between the resin layers is insufficient and, accordingly, the layers readily separate from each other; the annual ring pattern thereby obtained is not delicate and does not have such depth as natural wood; the apparatus required for the process is large in scale and is expensive; and complicated operations are required.